Studies are directed toward understanding the process by which regulation of the immune system is controlled at the cellular level. The co-operative roles of thymus derived (T) cells, both helper T-cells and suppressor T-cells are being delineated. We are seeking an understanding of the mechanism of interaction of these cells. We have shown that a streptococcal exotoxin (SPE) inhibits preferentially an activity which is presumed to be controlled by suppressor cells. SPE might therefore be a useful probe for the study of suppressor cells. Other standard reagents such as bacterial endotoxins, and various plant lectins such as phytohemagglutinins (PHA) are used to identify precursors of antibody secreting cells (B-cells) and helper T-cells respectively. Further cell separations are made by selective cytolysis with specific antisera and Guinea-pig-complement (C') and by various adherence techniques. Immunocytes from immunocompetent (plus/nu) donor mice selected in this way are tested in reconstruction experiments for their capacity to complement the immune responses of T-cell deficient Nude (nu/nu) mice or their spleen immunocytes in vitro. This study involving the use of SPE may also derive relevant information for a solution of the sequelae of Streptococcosis.